kernel memory parms

I am running Debian stable with postgresql 7.2.1. My machine has 4 GB of
RAM and I have configured the kernel to recognize it all. I have the
following shared mem options set:

kernel/shmmni = 4096
kernel/shmall = 3879731200
kernel/shmmax = 3774873600

Based on what I read in the docs I should be able to use at
setting of shared_buffers = 454000 in my postgresql.conf file as I want
to use about 3600MB of memory for my database.

However when I try to use that many segments I get the following error:

IpcMemoryCreate: shmat(id=1015811) failed: Invalid argument

The highest number of shared buffers it allows me to use is 258169. After
starting postgresql with the shared_buffers = 258169 and
max_connections = 64 ipcs shows the following:

wvcdl:/etc/init.d# ipcs

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 491520 root 600 33554432 6 dest
0x00000000 360449 root 600 92164 6 dest
0x00000000 294914 root 600 33554432 6 dest
0x0052e2c1 1867779 postgres 600 2144206848 3

------ Semaphore Arrays --------
key semid owner perms nsems status
0x0052e2c1 5734400 postgres 600 17
0x0052e2c2 5767169 postgres 600 17
0x0052e2c3 5799938 postgres 600 17
0x0052e2c4 5832707 postgres 600 17
0x0052e2c5 5865476 postgres 600 17

------ Message Queues --------
key msqid owner perms used-bytes messages

It looks to me as if there is about 2 GB of shared memory in use. Is this
a hard limit I can't go past? If so any sugestions on how I can make full
use of my mem for database use?

Thanks,
jth